1. Field of the Invention
The present invention is directed to a method for measuring the flow resistance of a catheter in an implanted medication infusion system for medication of the type having a medication reservoir and a pump that conveys the medication from the reservoir through a catheter connected to the discharge of the pump via a one-way valve into the patient, and having a rinsing input (port) for direct access to the catheter disposed upstream of the catheter and downstream of the pump. The invention is also directed to an apparatus for measuring flow resistance.
2. Description of the Prior Art
A problem associated with implanted infusion systems for delivering medications in the form of a liquid, for example an insulin solution, through a catheter to a patient is that flow impediments can arise in the catheter. Such flow impediments arise particularly in the catheter or in the area of the catheter opening. These flow impediments can occur because, for example, medication or body fluid leaking in a backward direction, or both of these fluids, can form deposits at the inside wall of the catheter. When these deposits become too thick, the delivery of medication is prevented. Flow impediments in the catheter, further, can arise when body tissue grows over or into the opening of the catheter.
It is desirable that the initial formation of such flow impediments in the catheter be discovered as early as possible so that measures that prevent a partial or total blockage of the medication delivery can be undertaken. Some form of measuring the catheter flow function is therefore desirable.
Heretofore, the catheter has been rinsed clean without making any flow measurement by inserting a syringe of a suitable size, for example 20 ml, by penetrating through the skin and through the septum of the rinsing input. The catheter is rinsed by manually discharging the contents of the syringe. The hope has been that the catheter flow function would thereby be improved. Such catheter rinsing is implemented "blind," i.e., without definite knowledge that rinsing was really necessary since no flow measurements were made. This is unsuitable since there are a number of reasons why an unnecessary rinsing of the catheter should be avoided. Further, no reliable information has been obtained in such known procedures as to whether the rinsing had the intended effect, or what, if any, modification of the flow function was achieved. The only "feedback" is that the syringe operator may possibly receive a subjective feeling that a blockage was dissolved.
An article entitled "Pompes Implantables A Insuline: Inter et d'un des 'port d'acces lateral pour le diagnostic et le traitement des obstructions di catheter," Selam appearing in Infusystemes, Vol. 6, pp. 30-32 (1989), discloses a method for indicating a blockage in a catheter by pressuring the catheter and registering the chronological decay of the pressure using a pressure transducer connected to the rinsing input, the pressure transducer being connected to a plotter. The result is compared to the shape of the decay curve of a new (unblocked and intact) catheter. Air bubbles in the syringe, in the pressure transducer and in the rinsing input cannot be entirely avoided, and this yields decay times of different length for the same catheter when the measurement is repeated. The measured results obtained with this technique are of a qualitative and not of a quantitative nature.